1. Field of the Invention
This invention relates to a process of decreasing the sulfur content of exhaust gases obtained during the recovery of sulfur from acid gases containing H.sub.2 S and other S-containing compounds in the Claus process, wherein the S-compounds contained in the tail gas are chemically reacted to form elementary sulfur and the sulfur is withdrawn.
2. Discussion of Prior Art
In a known process for decreasing the sulfur content of exhaust gas obtained during the recovery of sulfur in the Claus process, the tail gas is burned, the burnt residual gases are passed at elevated temperature through a coke layer and are then cooled, the cooled gases are reversibly taken up by liquid absorbents and SO.sub.2 is expelled and recycled to the Claus process (German Pat. No. 22 53 806).
On another known process of purifying the tail gases from the Claus process with recovery of sulfur, the tail gases are passed over active adsorbents at a lower temperature and the laden adsorbent is desorbed by a treatment with hot process gas. That process gas is circulated and the residual sulfur compounds are burned. The heat required to desorb the laden adsorbent is made available by a heat exchange between the desorbing gas and the exhaust gas that has been heated by the heat of reaction resulting from the reaction of the residual hydrogen sulfide with oxygen in the incinerator (German Pat. No. 25 55 096).
In another known process the tail gas from the Claus process is hydrogenated to form H.sub.2 S and the latter is reacted with SO.sub.2 to form sulfur. To that end, part of the hydrogen sulfide contained in the fresh acid gas is oxidized with a stoichiometric or slightly substoichiometric quantity of air to form sulfur dioxide and/or part of the resulting sulfur is reacted to form sulfur dioxide and the latter is reacted with the hydrogen sulfide formed by the hydrogenation. For this purpose, the still unreacted hydrogen sulfide is catalytically reacted with sulfur dioxide at temperature of 100.degree. to 150.degree. C. to form sulfur and the hot gases which contain hydrogen sulfide and are free from oxygen are passed through the laden catalyst to regenerate the latter (German Pat. No. 26 48 190).
The use of said known processes results in a higher yield of sulfur and in a purification of the tail gas from the Claus process to such a high degree that the exhaust gas can be delivered to the atmosphere.
On the other hand, the processes are complicated and consist of a plurality of steps. The energy consumption and capital requirement are considerable. Extraneous chemicals, such as coke, liquid absorbents, e.g., sodium phosphate solutions, or solid adsorbents, e.g., activated charcoal, are needed, which tend to be soiled and to lose their activity so that they must often be renewed. They can be regenerated only with difficulty. When it is desired to expel the absorbed SO.sub.2, the liquid must be heated to the boil. Sulfur-laden activated charcoal must be regenerated with insert gases at elevated temperatures.
The main disadvantage of all known processes resides in that the SO.sub.2 produced by a combustion or the high-H.sub.2 S exhaust gas obtained by the hydrogenation cannot be directly converted to S because the constituents H.sub.2 S and SO.sub.2 are not present in their stoichiometric ratio of 2:1. For this reason said gases are usually recycled to the Claus plant, in which a stoichiometric ratio of 2:1 of H.sub.2 S to SO.sub.2 is maintained by a suitable control of the rate at which combustion air is supplied to the acid gas burner. In a first approximation, the air rate is maintained in direct proportion to the rate of acid gas. The control is responsive to changes of the throughput rate of the acid gas but is not influenced by its composition. If the air rate is to be controlled also in dependence on the composition of the acid gas, all combustible constituents, not only the H.sub.2 S, must be taken into account so that the equipment required for measuring and controlling is considerably increased.
Because the hydrocarbons are not completely burned in the Claus process it is also necessary to monitor the ratio of H.sub.2 S to SO.sub.2 in the tail gas and to correct the rate at which air is supplied to the burner. In the prior art, the combustion air or part thereof is controlled in dependence on the ratio of H.sub.2 S to SO.sub.2 in the tail gas. This involves a great disadvantage, which resides in that there is a substantial delay between a change of the air supply to the burner and the effect of said change on the ratio of H.sub.2 S to SO.sub.2 in the tail gas. Owing to that delay, the feedback control system must have a slow response so that overshooting will be avoided.
As a result, the control system will respond only slowly to a change of the acid gas composition. For two reasons, it is not practicable to sample the tail gas at a point which is nearer to the combustion chamber so that the time between a change of the air rate and the detection of a change of the ratio of H.sub.2 S to SO.sub.2 could be shortened.
1. The concentration of H.sub.2 S and SO.sub.2 increase in that direction so that there is a smaller change of the ratio of H.sub.2 S to SO.sub.2. PA1 2. Secondary reactions taking place on Claus catalysts, such as the hydrolysis of COS or the reduction of SO.sub.2 by hydrogen, change the ratio of H.sub.2 S to SO.sub.2 in the Claus plant.
In the prior art it is not possible to obtain ratio of 2:1 of H.sub.2 S to SO.sub.2 in the tail gas. This fact results in losses of sulfur and in a larger emission. The deviation of the ratio of H.sub.2 S to SO.sub.2 from 2:1 will result in particularly high sulfur losses if the tail gas is desulfurized in that the Claus reaction EQU 2H.sub.2 S+SO.sub.2 .revreaction.3/nS.sub.n +2H.sub.2 O
is continued and the elementary sulfur is withdrawn.
It is an object of the invention to eliminate these and other disadvantages of the prior art and to provide a new way of decreasing the emission of sulfur from the Claus process of recovering sulfur.
The new process should permit an adjustment of the stoichiometric ratio of 2:1 to H.sub.2 S to SO.sub.2 in the tail gas before the latter enters a desulfurizing plant. The process should be inexpensive and require no extraneous chemicals and involve only a low structural expenditure and low energy costs.